Cathode for electron discharge devices



April 1941- w. EHRENBERG 2,239,416

CATHODE FOR ELECTRON DISCHARGE DEVICES Filed Jan. 24, 1940 7 INVENTQR. WERNER EHRENBERG ma ATTORNEY.

Patented Apr. 22, 1941 CATHGDE FOR ELECTRON DISCHARGE DEVICES Werner Ehrenberg, Hayes, England assignor to Electric & Musical Industries Limited, Hayes, Middlesex, England, a company of Great Britain Application January 24, 1940, Serial No. 315,375 In Great Britain January 25, 1939 Claims.

lhis invention relates to cathodes for electron discharge devices, such as cathode ray tubes, X- ray tubes and other apparatus. The invention is essentially concerned with a type of cathode known as an impact cathode in which a plate or disc of metal is heated to incandescence so as to become electron-emissive under the action of a bombarding stream of electrons. This type of cathode possesses advantages over the usual indirectly-heated oxide cathode or the directlyheated metallic filaments such as are usually employed where it is desired to obtain a concentrated electron stream.

In British Patent Specification No. 338,480 an impact cathode is proposed in which a flat metal .plate of suilicient thickness to assume a uniform temperature is arranged in front of a heating filament. It is suggestedin this specification to connect the emitting plate to a focussing member surrounding the plateby small tabs which project across a gap between the plate and the tocussing member. When such a cathode is heated, however, the expansion of the tabs is likely to cause the emitting plate to be deflected from .its axis and may also cause said plate to become distorted. Further, the said plate may also becomedistorted due to its own expansion, particularly if the distribution of the bombarding electrons is not uniform.

It is the object of the present invention to provide an improved impact cathode in which the aforementioned disadvantages are overcome and which can readily be manufactured.

According to the present invention, there is provided an impact cathode comprising a metal plate which either carries or-itself constitutes an emissive surface, said plate having two or more arms projecting therefrom by which it is supported from a supporting member, said arms being bent out of the mean plane of said plate, the construction being such that the axis of said emissive surface is not changed when said arms expand.

The metal plate is preferably dished so that when it expands, buckling is avoided.

The cathode is preferably fed from a source of voltage, the D. C. regulation of which is so arranged that the maximum power output of said source is substantially equal to the power which has to be dissipated in said plate to raise it to the desired temperature so as to minimise variations in the temperature of said plate due to variations of the voltage of said source.

In order that the invention may be more clearly understood and readily carried into effect, alternative forms of cathode constructed in accordance with the invention will now be described in greater detail by way of example with reference to the accompanying drawing.

Figure l is an exploded view illustrating the various parts of the cathode according to the preferred form of the invention, and

Figure 2 is a longitudinal section showing parts of Figure 1 in an assembled condition.

Figures 3 and 4 illustrate diagrammatically two forms of the emitting plate in accordance with the invention,

Figure 5 illustrates a circuit arrangement of a suitable power unit for operating an impactheated cathode and Figure 6 is a graph for illustrating the regulation of the power unit in Figure 5.

As shown in Figures 1 and 2, the cathode proper comprises a disc 3 of suitable material, such as tantalum or columbium, provided in the example shown with three integral arms 4 which do not lie in the plane of the disc 3. For example, the arms 4 may be bent adjacent the disc 3 at an angle of about 45 thereto and also bent again at 45 at their extremities where the arms are attached as, for example, by welding to the interior of a support 5, the upper end of the support 5 being closed except for an aperture 6 in which the disc 3 is positioned in spaced relation therefrom. The disc 3 is about four thousandths of an inch thick and is mounted centrally of the support 5 which is preferably made of nickel. The heater comprises a spiral heating element I, which is surrounded by a shield 8, the heater 1 and shield 8 being supported'from a base 9 by three support rods lflprojecting from the base 9 and insulated therefrom. One end of the heater 1 is electrically connected to the shield 8. The support 5 and the heater 1 are heldin the required relationship, as shown in Figure 2 by a sleeve I I over the upper end of which is fitted the support 5, whilst the lower end of the sleeve I l is connected to the base 9. The upper end of the sleeve II is provided with longitudinal slots [3, as shown, each of which permits the arms 4 to be maintained out of contact with the sleeve II when in thev assembled condition, whilst the lower end of the sleeve H is provided with'lugsv it which are adapted to be secured to lugs I4 projecting upwardly from the base 9 as by weldin The diameter of the sleeve H is larger than the diameter of the shield 8 so as to leave a suitable clearance between the upper end of the sleeve I l and the shield 8. The diameter of shield 8 is also preferably smaller than the diameter of disc 3 The support 5 may be secured to the sleeve H in any suitable manner, as by tabs, not shown :or by welding; or it may be fixed inside ametallic annular flange which may be secured to the sleeve H and may also serve to aid in the cooling of the support 5 and be provided with support rods which may support the cathode structure-and in the case of cathode ray tubes elements of the cathode ray gun. v Asjshomi in Figures 3 and 4, the plate is dished so that'the' emitting surface is slightly concave or slightly convex respectively, so that when the plate is heated, the deformation which is likely to occur when the plate is flat due to expansion, is avoided as the effect of the expansion of the dished plate is merely slightly to change its curvature. The construction is specially adapted for use in cathode ray tubes since the curvature of the emitting surface may be chosen in known manner to reduce certain aberrations of the electron optical system of the tube.

The long relatively thin arms 4 supporting the disc 3, ensure that the emitter is mechanically stable and when the emitter is heated the arms serve to transfer any expansion into a purely axial movement of the emitting disc 3. The arms 4 also serve to minimise heat losses. It is, however, relatively unimportant how many arms 4 are employed so long as a sufficient number is used to preserve mechanical rigidity and it is relatively immaterial how the arms 4 are bent and whether the emitter 3 is of circular shape. Preferably, however, three arms 4 are employed which are equi-distantly spaced around the circumference of a circular emitter 3 as shown. Although it is preferred to use tantalum or columbium for the disc 3, other materials giving good thermionic emission such as tungsten or zirconium may, however, be used. Alternatively, the disc 3 may be replaced by a ring of a suitable refractory metal, such as tantalum, molybdenum or tungsten, there being inserted in the aperture in the ring a piece of metal giving very high thermionic emission, such as columbium.

The provision of the lugs t3 which, when the sleeve H is connected to the base 9 leaves apertures round the base 9, prevents overheating of the base 9 and also enables small adjustments of the filament assembly to be made after the parts are assembled. In some cases, however, the sleeve l I may be omitted and the support 5, in this case, may be directly connected to the filament assembly. In this case, the support 5 may be sufficiently long substantially to enclose the filament assembly, or the support 5 may be provided with depending lugs for connection to the lugs 14.

The base 9 may be made of metal in which case the support rods iii may be insulated therefrom by the provision of glass beads which seal the support rods ID to the base 9. or, alternatively,

a ceramic or mica base 9 may be employed, suitable alterations in construction being made to enable the sleeve I! or the support 5 to be connected to the ceramic or mica base.

In an alternative construction, the wires l6 and the lugs l4 may be moulded into a base of ceramic material. The wires H), which may be flattened where they are to pass through the ceramic material, in order to lock them positively therein and the lugs 14, which are preferably bent so as to provide tongues adapted to engage with the ceramic material, are first positioned in a die. The compound which is to be fired to form the ceramic material is then pressed into the die in a moist state, and the wires and lugs bound by the compoimd, are then removed from the die and fired at a high temperature. The filament 1 is preferably a single tungsten spiral, although a double spiral may be employed if desired and the filament made of a different material, such as tantalum or colubium. The shield 8 serves the purpose of focussing the electrons emitted by the filament I to the centre of the disc 3 so preventing a lateral spread of the electrons. The shield 8 is preferably substantially cup-shaped, but

iii)

supported in any suitable manner.

may, in some cases, be in the form of a cylinder The shield 8 is preferably made of tantalum but other refractory metal may, if desired, be employed.

The improved cathode may be used as the emitting cathode of any type of electron discharge device where it can be suitably employed,

but has particular use in cathode ray tubes such as are used for the reconstitution of television images, the cathode ray tubes in question being of the projection type, in which a small but very bright picture is required to be produced for projection by an external optical system. In these tubes a cathode having a high emission is required. It will be appreciated that since the emitting disc under operating conditions becomes incandescent, a certain amount of light will be emitted by the disc 3 onto the screen of the tube. Whilst this might be regarded as a disadvantage in the so-called direct vision cathode ray tubes, it is negligible in projection tubes not only on account of the higher level of brightness at which these tubes are operated, but also, since the screen of a projection tube is thicker than those of direct vision tubes, any light which reaches the screen from the disc 3 is less liable to be visible from the front of the screen.

The impact cathode used in carrying out the invention requires for its operation a low tension voltage for energising the filament 1 in Figure 5 and a high tension voltage for accelerating the electrons, the high tension voltage being applied to the support 5. These voltages can be supplied from any source that is in a suitable potential relation to the other parts of the electron gun to which the cathode is applied. The preferred form of voltage source, however, is a separate power unit that supplies an A. C. voltage for heating of the filament 1 and a D. C. voltage for application to the support 5, the regulation of the D. C. supply being such that its maximum power output is obtained at or near the rated emission current from the filament I.

The emission of the tungsten spiral varies very rapidly with the voltage applied to it. A change of heater voltage of about 4 per cent increases the emission by about one-third in the operating range. If the power unit supplying a constant H. T. voltage is used for accelerating the electrons the wattage loaded on the emitter changes to the same extent. It is, however, necessary to keep the load on the emitter constant within a few per cent. for an efficient performance since its life (limited by evaporation) and its emissivity vary rapidly with the degree of heating. This diificul-ty can, however, easily be overcome by using a power unit with a suitable regulation.

The regulation of an electric supply can usually be represented as a resistance, and it is well known that a voltage source of an open circuit voltage V and regulation resistance R has a maximum power output at a current of At a lower and higher current the wattage supplied by the power unit decreases, but the maximum is rather flat. A power unit regulated so as to have thepower required by the cathode as its maximum power output at the rated emission current loads the cathode, therefore, in first approximation independently of the voltage applied to the filament or the emission of the filament.

A suitable power unit is illustrated in Figure and a graph showing the performance of such a power unit is illustrated in Figure 6. A full wave rectifier valve I5 is employed energised by a transformer I6 having three secondary windings, the first winding I 1 being a sixvolt winding, the second winding I8 being centretapped, each half being a 500 volt winding, and the third winding l9 being a four-volt winding. The power unit shown in Figure 5 may be designed to operate an impact-heated cathode requiring an electron current of about 90 milliamperes and a loading of about 30 watts. Between the anodes and the cathode of the rectifier IS the usual smoothing condensers and 21 are employed which may each have a. value of about 4 microfarads, but instead of employing a smoothing choke, as is usual, a resistance 22 is employed which may have a value of about 2500 ohms. This value of resistance is such as to give the power pack the required regulation. The filament l is heated by alternating current derived from the winding I! through a resistance 23 which is adjustable to afford the required filament voltage.

Figure 6 is a diagram showing how the accelerating voltage applied to the cathode and the cathode loading varies with the filament emission. It can be seen that the voltage drops rapidly With increasing current so counteracting the increase of current. Between about 72 and 112 ma. the wattage does not vary from a loading of 29.5 watts by more than about 0.5 watt. Mains fluctuations usually affect the H. T. output to the same degree as the filament. Since the influence of the fluctuations with respect to the filament heating is practically eliminated, the actual wattage on the cathode follows approximately proportionally the fluctuations of the mains voltage. The efiect of mains fluctuations are thus kept within tolerable limits.

The regulation of a power unit depends on its design and the values of the components used. Any type of power unit can be so arranged as to have the regulation required, either by inserting resistances in any part of the circuit A. C. or D. C. which carries the power for the acceleration of the electrons, and/or by suitably dimensioning the components that affect the regulation, as iron core of transformer, type of rectifier valve, value of condensers, etc.

Although the forms of impact cathode described are intended for use particularly in cathode ray tubes of the so-called projection type, they may of course be employed in tubes of the direct vision type or in any other form of electron discharge device.

I claim:

1. A cathode assembly comprising a metal plate adapted to be heated by electron bombardment, said plate having at least three integral portions of relatively small cross-section to minimize heat loss, an apertured disc, a flange shield serving as the sole support for said apertured disc and said metal plate and supporting said plate and said disc through said integral portions in substantially coplanar relation, and means within the enclosure formed by said disc and said flange shield to liberate electrons to impinge upon and heat said metal plate.

2. A cathode assembly including a metal plate adapted to be heated by electron bombardment having at least three integral portions bent away from the plane of said plate, means to liberate electrons to impinge on and heat said plate, and means to confine the liberated electrons to the space between said electron liberating means and said metal plate comprising a flanged apertured member electrically connected to and being the sole mechanical support for said metal plate through said integral portions such that the aperture of said member encircles said plate substantially in a common plane.

3. A unitary cathode assembly comprising a metal plate having at least three integral portions of small heat conducting cross-section bent away from the plane of said plate, electron emissive means to liberate electrons to impinge on and heat said plate, a flanged apertured member surrounding the space between said plate and said electron emissive means and mechanically supporting said plate through said integral portions to shield regions exterior of said flanged apertured member from electrons liberated from said emissive means, a plurality of current carrying leads connected to said electron emissive means, a support base supporting said leads in electrically insulated relationship, and a metallic sleeve member constituting the sole support of said metal plate and said flanged apertured member from said base.

4. A unitary cathode assembly comprising a support base, a plurality of current carrying leads insulatingly supported by said base, electron emissive means supported by said leads, a cup-shaped member surrounding said emissive means to concentrate the electron emission therefrom, a metal plate in the path of electrons from said emissive means, said plate having at least three integral portions, a flanged apertured member supporting said metal plate through said integral portions and a metallic sleeve constituting the sole support for said metal plate and said flanged apertured member from said support base, to space the said metal plate from said electron emissive means.

5. A self-contained unitary cathode assembly comprising a support base, a plurality of current carrying leads extending through and insulatingly supported by said base, a filamentary electron emitter supported by said leads, a shield surrounding said emitter to direct electrons therefrom in a direction away from said base, a metal plate adapted to emit electrons when bombarded by electrons from said emitter, said plate having at least three integral tabs for support thereof, an apertured disc and a flange member surrounding said metal plate, the said tabs being bent from the plane of said metal plate and afiixed to said flange member, and a tubular shield aflixed to said support base at oneend and to said flange member at the opposite end thereof and abutting said apertured disc to accurately space said disc and said metal plate from said emitter.

WERNER EHRENBERG. 

